1. Field of the Invention
The present invention has for its object devices and processes for the heat insulation of least one underwater pipe at great depth.
The technical sector of the invention is the domain of the manufacture and assembly of an insulation system outside and around the pipes in which hot effluents, of which it is desired to limit heat losses, circulate.
This invention is applied more particularly to the developments of deep-sea oil fields, i.e. oil rigs installed on the open sea, in which the surface equipment is generally located on floating structures, the well heads being on the sea bed. The pipes concerned by the present invention are either links between well heads, or the part resting on the bed of the bed/surface links.
2. Description of the Related Art
Deep-sea developments are at the present time effected at water depths reaching 1500 meters. Future developments are envisaged at depths of up to 3000 meters and more.
The principal application of the invention is the heat insulation of pipes or lines immersed under water and more particularly at great depth, beyond 300 meters, and conveying hot oil products of which too great a cooling would be problematic both during normal production and in the case of production being stopped.
In effect, in this type of application, numerous problems are raised if the temperature of the oil products decreases by a significant considerable value with respect to their production temperature which is generally beyond 60 to 80° C., while the temperature of the surrounding water, especially at great depth, may be less than 10° C. If the oil products cool for example below 30° to 60° C., for an initial temperature of 70 to 80° C., the following is generally observed:                a considerable increase in the viscosity which then reduces the flowrate of the pipe,        a precipitation of dissolved paraffin which then increases the viscosity of the product and of which the deposit may reduce the useful internal diameter of the pipe,        the flocculation of the alphaltenes inducing the same problems,        the sudden, compact and massive formation of gas hydrates which precipitate at high pressure and low temperature, thus suddenly obstructing the pipe.        
Paraffins and alphaltenes remain attached to the wall and then require cleaning by scraping the inside of the pipe; on the other hand, the hydrates are still more difficult, and even impossible to remove.
The function of the heat insulation of such pipes is therefore to delay cooling of the oil effluents conveyed, not only during established production for their temperature to be for example at least 40° C. on arriving at the surface, for a production temperature at the entrance of the pipe of 70° C. to 80° C., but also in the case of reduction or even stoppage of production in order to avoid the temperature of the effluents descending for example below 30° C. in order to limit the problems mentioned above or at least allow them to be rendered reversible.
Moreover, when such pipes are to be laid at depths greater than 300 meters, the ambient pressure of at least 30 bars prevents the use of high-performance heat insulators which are encountered on land or at shallow immersion, as they all use gases of which the heat conductivity is in effect very low and whose convection is blocked by a solid, porous, cellular or fibrous material: however, the compressivity of the gases does not allow these conventional heat insulators to withstand high outer pressures.
Japanese Patent Application No. JP2176299 published on Oct. 25, 1991 might also be cited, which describes a device for insulating metallic or synthetic resin tubes for supplying hot water in buildings and of which it is desired to conserve the temperature at more than 50° C. after one hour of stoppage of supply of hot water, in an ambient temperature of 13° C. for example: to that end, it describes a structure comprising a tube for the circulation of hot water, which is preferably deformable to facilitate laying thereof, with a layer of porous material imbibed with paraffin to about 200% and covering it, and another layer of refractory material covering the periphery of the assembly; the use of paraffin makes it possible to have an advantageous coefficient of heat insulation although less than the heat insulators mentioned above and comprising gas, but the capacity of heat accumulation of this Japanese device is reinforced by the presence of the outer refractory layer making it possible to reduce heat loss with the advantage of being able to cut the whole of this structure at any spot in order to facilitate assembly thereof and without loss of the heat accumulation power. However, such a solution cannot be used in water, especially at great depth where it is necessary to be able to withstand a considerable outer hydrostatic pressure, while ensuring sufficient containment in order to avoid any risk of pollution and/or loss of thermal efficiency. Moreoer, it does not contribute the specific characteristics described and claimed in the present invention.
Moreover, other specific types of heat insulation compatible with deep immersions have rathermore been developed, which may be grouped in three families, namely:                the outer coatings made of solid plastic such as polyurethane, polyethylene, prolypropylene . . . but whose heat conductivity is fairly average since of the order of 0.2 to 0.3 Watt/meter/degree Celsius, which may be sufficient in continuous production but insufficient to preserve a minimum temperature for a given time in the event of stoppage of production,        the coatings made of syntactic materials constituted by hollow balls containing a gas and resistant to the outside pressure and embedded in various binding agents such as concrete, epoxy, elastomer, polypropylene, etc . . . : the ones with highest performance are the syntactic materials based on epoxy binding agent and on hollow glass microspheres of fairly low conductivity and interesting since of the order of 0.10 to 0.15 watt/meter/degree Celsius, but the cost of these coatings is very high,        the “pipe in pipes” in which a first inner tube conveying the effluents is disposed concentrically in a second tube resistant to the outside hydrostatic pressure; the annular space included between the two tubes may either be filled with heat insulator with very low heat conductivity (0.02 Watt/meter/degree Celsius) and which, in order not to be crushed, must be left at atmospheric pressure, or a vacuum may be created therein: such a solution necessitates partitions disposed longitudinally and perfectly tight, at regular intervals, for questions of safety, and complicates the construction and positioning of such assemblies which are, moreover, very expensive.        
Another technique consists in prefabricating shells of syntactic foam and in assembling them around the pipe or in making a continuous coating of syntactic foam around said pipe. We would recall on this subject that the syntactic foam is constituted by hollow microspheres containing a gas and bonded by a resin generally of the epoxy type.
These deep-sea insulation technologies use very high-performance products which are extremely expensive and difficult to employ on a large scale.
In the case of installing single pipes or so-called bundles of pipes, it is generally preferred to manufacture said pipes on land in unitary lengths of 500 to 1000 m which are then
pulled from the open sea with the aid of a tugboat. In the case of pipes of several kilometers, the first length is pulled, which is joined to the following, the tugboat maintaining the whole in traction during the joining phase, which may last several hours. When all the pipe or bundles of pipes has been put in the water, the whole is towed, generally rubbing on the sea-bed, towards the site, where it is then placed in position.
The insulation of the pipe or pipes or of the bundle is then protected by an outer envelope which has a double function: —on the one hand that of avoiding damage which might occur during towing, which may in certain cases take place over distances of several hundreds of kilometers, which requires using fairly resistant materials such as steel, thermoplastic or thermosetting compound or a composite material; —on the other hand, that of creating a containment around the insulation system.
Such containment is necessary in the case of outer insulating coatings constituted by shells of syntactic foam assembled around the pipes, as the interstices existing between the various shells, as well as the space between the shells and the outer envelope, are filled with a virtually incompresssible product, which is generally fresh water or passivated sea water or any other product compatible with the internal components.
In effect, with sea-beds of 2000 m, the hydrostatic pressure is of the order of 200 bars, or 20 Mega Pascals, which implies that all the pipes and the insulating system thereof must be capable of withstanding not only these pressures without degradation during pressurizations and depressurizations of the pipe in which the hot fluid circulates, but also the temperature cycles which will generate variations in volume of the different components as well as of the interstitial fluids, and therefore positive or negative pressures which may lead, if the outer envelope is tight, to partial or total destruction thereof either by exceeding the stresses admissible, or by implosion of this outer envelope (negative variations of internal pressure).
If said outer envelope is not tight, the assembly will then be at equal pressure with respect to the outer pressure, but this will then result in exchanges of fluids between the interior of the bundle and the outer medium. In the case of a filling of the interstices of the bundle with fresh water, passivated sea water or any other product compatible with the internal components as indicated hereinabove, as it is in that case sought to avoid fluid exchanges with the outer medium, one is led to arrange bags constituted by a supple membrane of elastomer type making it possible to contain the variations in volume by maintaining the variations in pressure at a reasonable level but these bags then complicate assembly of the insulating device and do not enable the stresses to be distributed in uniform manner.
The problem raised is therefore that of being able to produce an insulation of at least one underwater pipe intended to be laid on the sea-bed in particular at great depth, of which the insulating coating can withstand not only the hydrostatic pressure but also all the efforts associated with its own weight, and induced upon laying during which the pipe undergoes frictions and is exposed to risks of punching; said insulating coating must make it possible to maintain for example a hot effluent such as an oil product produced for example at 60° C. at the level of the sea-bed, at a temperature above for example 40° C. when it arrives on the surface after a distance of several kilometers in the water, and, moreover, to maintain a temperature at more than 30° C. for example even after several hours of stoppage of production, and this with manufacturing costs which are less than those of present syntactic materials, while offering various possibilities of implementation, and this without risk of pollution for the environment.